1. Field of the Invention
This invention relates to a process for separation of aromatic compounds from a hydrocarbon feed, and more particularly to a supported liquid membrane process for aromatic separation.
2. Description of Related Art
Aromatic compounds such as benzene, toluene and xylene have significant commercial value as intermediate products for use in many commodity chemicals and polymers, including styrene, phenol (via cumene), and cyclohexane. Useful products derived from these aromatics include nylons, polyurethanes, polyesters, resins, and plasticizers.
Nonetheless, until recently, there has been little incentive to undertake the expense and effort to remove aromatics, e.g., benzene, from one of its most abundant sources, cracked naphtha. However, new regulations passed by the Unites States Environmental Protection Agency require that the benzene content of gasoline be reduced to an annual refinery average of 0.62 volume % by 2011. Many other nations have followed suit to minimize the health impacts that benzene has on humans and animals as a known carcinogen.
Separation of aromatic compounds from hydrocarbon mixtures containing aliphatic and aromatic compounds has been practiced for considerable time. Particular care must be taken with compounds that have similar properties, e.g., boiling points. Ordinary distillation is generally ineffective to separate aromatics from azeotropes, i.e., mixtures in which aliphatic compounds have boiling points similar to the aromatics. Certain compounds can be separated using azeotropic distillation, however, this process requires adding a third component to generate another lower-boiling azeotrope, and another separation is needed to remove the newly-introduced component.
Liquid-liquid extraction is another type of process employed to separate aromatics from hydrocarbon mixtures. In general, liquid-liquid extraction of aromatics employs a solvent to phase-separate an extract phase containing a target aromatic species, and a raffinate phase containing the unextracted material. However, separation of the aromatics from the hydrocarbon mixture, and the salts from the extract phase, can often prove difficult and process-intensive.
Membrane separation is another technique used to remove aromatics from a hydrocarbon mixture that can result in more complete separation without the subsequent solvent removal steps associated with liquid-liquid extraction. Existing membrane processes typically rely upon pervaporation or perstraction conditions under both elevated temperatures and pressure differentials to remove aromatics from hydrocarbons or naphtha streams. The types of membranes currently available and used in these separation processes include those based on solid polymers and ceramics.
In general, existing membrane separation technology is not effective to remove aromatics from hydrocarbon streams with a high degree of selectivity at a relatively low cost. Therefore, a need exists for an aromatic separation system and process for efficiently and effectively removing aromatics from a hydrocarbon feed containing a mixture of aromatic and non-aromatic hydrocarbons.
It is therefore desirable to provide an aromatic separation system and process that results in a benzene-lean gasoline product stream. It is also desirable to provide an aromatic separation system and process that allows for the recovery of valuable aromatics.